1. Field of the Invention
The present invention relates to power supply circuits. More specifically, the present invention relates to a power supply circuit which is particularly suitable for supplying a voltage to, for example, a voltage-controlled oscillator in a cellular phone.
2. Description of the Related Art
FIG. 4 is a circuit diagram of a conventional power supply circuit. The power supply circuit includes a voltage stabilizer circuit 31 and a ripple reducing circuit 32, each implemented in the form of an integrated circuit. The voltage stabilizer circuit 31 has a first voltage input terminal 31a, a first voltage output terminal 31b, and a first control voltage input terminal 31c. When a high-level control signal is applied to the first control voltage input terminal 31c, a voltage input to the first voltage input terminal 31a is stabilized to a particular voltage, for example, 3 volts, before being output from the first voltage output terminal 31b. The first control voltage input terminal 31c is connected to a power supply voltage input terminal 33. The power supply voltage input terminal 33 is supplied with a voltage, for example, 3.6 volts, from a power supply (not shown), for example, a battery.
The ripple reducing circuit 32 has a second voltage input terminal 32a, a second voltage output terminal 32b, and a second control voltage input terminal 32c. When a high-level control voltage is applied to the second control voltage input terminal 32c, a voltage input to the second voltage input terminal 32a is caused to have reduced ripple before being output from the second voltage output terminal 32b. The second control voltage input terminal 32c is connected to the power supply voltage input terminal 33. The second voltage input terminal 32a is connected to the first voltage output terminal 31b of the voltage stabilizer circuit 31. The second voltage output terminal 32b is connected to a load voltage output terminal 34. The load voltage output terminal 34 is connected to a load (not shown), for example, a voltage-controlled oscillator.
In accordance with the above configuration, the minimal voltage drop between the first voltage input terminal 31a and the first voltage output terminal 31b of the voltage stabilizer circuit 31 is on the order of 0.15 volt. Thus, when the power supply voltage is 3.15 volts or higher, a voltage of 3 volts is constantly output from the first voltage output terminal 31b; however, when the power supply voltage is below 3.15 volts, a voltage which is 0.15 volt lower than the power supply voltage is output from the first voltage output terminal 31b. 
The minimal voltage drop between the second voltage input terminal 32a and the second voltage output terminal 32b of the ripple reducing circuit 32 is on the order of 0.25 volt. Thus, a voltage which is 0.25 volt lower than a voltage input to the second voltage input terminal 32a is output from the second voltage output terminal 32b. 
When the power supply circuit is used in, for example, a cellular phone, the battery is often used for a period long enough to render the output voltage of the voltage stabilizer circuit 31 lower than the rated voltage (3 volts), for example, as low as 2.5 volts.
When the voltage stabilizer circuit 31 outputs a voltage below the rated voltage, the voltage stabilizer circuit 31 diminishes its ability to reduce ripple, as shown in FIG. 5, increasing ripple which enters the ripple reducing circuit 32. Then, the ripple reducing circuit 32 fails to sufficiently remove the ripple, thus outputting a voltage containing considerable ripple to the load voltage output terminal 34.
Consequently, C/N ratio decreases for signals generated by the voltage-controlled oscillator (or other types of signal generator) connected to the load voltage output terminal, thereby degrading performance of the cellular phone.
Accordingly, it is an object of the present invention to provide a power supply circuit in which the ripple reducing ability is maintained even when an output of a voltage stabilizer circuit in the power supply circuit drops below a rated voltage due to a decrease in power supply voltage.
To this end, the present invention provides a power supply circuit including a power supply voltage input terminal; a load voltage output terminal; a voltage stabilizer circuit having a first voltage input terminal and a first voltage output terminal, which stabilizes a voltage input to the first voltage input terminal and which outputs a rated voltage from the first voltage output terminal; and a ripple reducing circuit having a second voltage input terminal and a second voltage output terminal, which reduces ripple in a voltage input to the second voltage input terminal and outputs from the second voltage output terminal. In the power supply circuit, the first voltage input terminal and the second voltage input terminal are connected to the power supply voltage input terminal, while the first voltage output terminal and the second voltage output terminal are connected to the load voltage output terminal. The voltage stabilizer circuit is activated when a power supply voltage input to the power supply voltage input terminal is a predetermined value or higher, while the ripple reducing circuit is activated when the power supply voltage is lower than the predetermined value.
Preferably, in the power supply circuit, the voltage stabilizer circuit further has a first control voltage input terminal to which a high-level control signal for activating the voltage stabilizer circuit is applied, while the ripple reducing circuit further has a second control voltage input terminal to which a high-level control signal for activating the ripple reducing circuit is applied. The power supply voltage input terminal and the first control voltage input terminal are connected via a first switching device, while the power supply voltage input terminal and the second control voltage input terminal are connected via a second switching device. The first switching device is turned on and the second switching device is turned off when the power supply voltage is the predetermined value or higher, while the first switching device is turned off and the second switching device is turned on when the power supply voltage is lower than the predetermined value.
More preferably, the power supply circuit further includes a voltage divider circuit for dividing a voltage output from the first voltage output terminal to produce a divided voltage. The first switching device is implemented by a first PNP transistor and the second switching device is implemented by a second PNP transistor, the emitters of each of the first and second PNP transistors being connected to the power supply voltage input terminal, the collector of the first PNP transistor being connected to the first control voltage input terminal and to the base of the second PNP transistor, the collector of the second PNP transistor being connected to the second control voltage input terminal. The divided voltage is applied to the base of the first PNP transistor, the divided voltage being so determined that the first PNP transistor is turned on when the power supply voltage is the predetermined value or higher and is turned off when the power supply voltage is lower than the predetermined value.
More preferably, in the power supply circuit, the predetermined value of the power supply voltage is the sum of the rated voltage of the voltage stabilizer circuit and a voltage drop between the second voltage input terminal and the second voltage output terminal of the ripple reducing circuit.
Thus, even when the power supply voltage drops to such an extent that the voltage stabilizer circuit no longer outputs a rated voltage, a voltage substantially free of ripple is output from the ripple reducing circuit, which serves to expand the life of a battery for the power supply. In accordance with the decrease in the power supply voltage, the voltage stabilizer circuit and the ripple reducing circuit are automatically switched therebetween for operation, without changing the output voltage of the power supply circuit at the time of switching.